In order to form interconnection circuits in a substrate such as a semiconductor wafer, there has been employed a process of plating a substrate with copper to form a copper layer thereon, and then removing unnecessary portions of the copper layer by chemical mechanical polishing (CMP) to form a copper interconnection layer in the substrate. During the chemical mechanical polishing (CMP) in such a process, it is necessary to accurately control the progress of the polishing process of the copper layer (the film thickness of the copper layer). An optical sensor or an eddy current sensor may be employed to control the film thickness of such a conductive film.
An optical sensor measures the film thickness of a conductive film based on the wavelength of light. Accordingly, it is possible to perform accurate measurement of the film thickness in areas to which light is applied. For example, in order to detect the film thickness of a conductive film formed on a semiconductor substrate during chemical mechanical polishing, it is necessary to form an opening at a position through which light is applied directly to the substrate during polishing. However, such formation of an opening may adversely affect polishing performance of the chemical mechanical polishing apparatus.
An eddy current sensor measures the film thickness of a conductive film based on the magnitude of an eddy current produced in the conductive film. Accordingly, for example, an eddy current sensor can be mounted in a polishing tool of a chemical mechanical polishing apparatus. Further, an eddy current sensor can measure polishing conditions of a semiconductor substrate in a non-contact manner during polishing. Thus, the polishing tool is not required to have an opening to observe the substrate. For example, the film thickness (the amount of polishing) of a conductive film formed on a semiconductor substrate can be measured in a non-contact manner during polishing without any windows being formed in a polishing pad. Furthermore, there has been proposed an eddy current sensor capable of detecting an endpoint of a polishing process of an ultrathin film such as a barrier layer.
However, when a sensor coil of an eddy current sensor is embedded in a polishing table made of a conductive material such as stainless, a magnetic flux generated by the sensor coil produces an eddy current in the polishing table. Accordingly, a magnetic flux externally emitted is lessened. In such a case, since a magnetic flux that reaches a semiconductor wafer to be measured is lessened, an eddy current produced in the conductive film on the wafer is also reduced. Accordingly, an eddy current sensor is required to have a high sensibility.
For example, in a case where an eddy current sensor is provided in a polishing apparatus, a polishing pad is disposed between a conductive film of a semiconductor substrate and a sensor coil of the eddy current sensor. If measurement environment such as the thickness of the polishing pad changes, then signals from the eddy current sensor also change. Thus, the eddy current sensor is required to measure the film thickness of the substrate in consideration of such measurement environment as one of parameters. Accordingly, complicated and troublesome processes are required to measure the film thickness of the substrate.
When an ultrathin conductive film is measured, an eddy current produced inside of a substrate such as a semiconductor wafer is not negligible. Accordingly, in order to measure the film thickness of the ultrathin conductive film, influence from the interior of the substrate should be considered as a measurement environment which would adversely affect measurement with the eddy current sensor. Thus, complicated and troublesome processes are required to measure the film thickness of the substrate.